Film-forming base for baking enamels



Patented Aug. 30, 1949 FILM-FORMllgg BASE FOR BAKING AMELS 'Arthur E. Young and Harold M; Hoogsteen, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application March 8, 1947, Serial No. 733,372

2 Claims.

This invention relates to compositions of matter for use in baking enamels of the synthetic, or non-vitreous type, and is concerned particularly with a speciflc combination of film-forming constituents to serve as a base for such enamels.

Because of their toughness and resistance to attack by alkalies, various compositions containing ethyl cellulose have been suggested for use in alkali-resistant coatings. It has also been suggested to modify such coatings by incorporating a thermostat resin in the composition, to harden the same when baked. The problem' remains, however, to provide such a composition which can carry with it the pigments commonly desired in synthetic enamel type coatings and will also possess all of the essential characteristics of adherence to metal surfaces, flexibility and hardness.

A coating for metals, to be deemed satisfactory for many purposes, must show no cracks in the flexibility test described in A. S. T. M. D-522-41, in which a thin metal strip coated with the test composition is bent about a standard conical mandrel.

Adherence of the coating to metal must be of a high order if the coating is to be deemed acceptable for many uses where coated articles are normally subject to considerable physical abuse.

A test for adherence which has been found to distinguish accurately between satisfactory and unsatisfactory coatings is that reported at page 59, "Ethocel Handbook, published in 1940 by The Dow Chemical Company. This test consists in applying the coating to metal, cutting through the coating with a razor blade in a crisscross pattern to form squares about inch on a side, and

. determining the relative ease or difficulty with which the flakes of coating can be removed from the metal. When the flakes fall off by themselves, the adherence of the coating is rated very poor. If they remain on the panel but can be removed easily with the thumbnail, the.- adherence of the coating is rated poor. If considerable pressure is required to remove the squares from the metal with the thumbnail, the adherence of the coating is rated fair. If a knife blade is required to remove the squares from the metal, the adherence is rated good. I

Hardness of a coating may be measured in any of several ways. Each method gives results which are comparable with those of other tests usin the same method, but some of them, such as the "pencil hardness test, are materially influenced by the individual technician. Tests for hardness which are practically independent of the personal factor include the "Taber" shear-hardness test and the "Sward hardness. test. For the p poses of the present application, the hardness values reported are those obtained by the Taber method, using attachment E-3'720 on the Taber Abraser, Model E1010, manufactured by the Taber Instrument Company, North Tonawanda, New York. The method is described at pages 20 and 21 in "Instruction Manual, 1945, published by the Taber Instrument Company. The measurement is made by mounting the coated metal on the turntable of the abraser, and bringing to bear against the coating a special wedge-shaped cutting instrument mounted below a pivoted and graduated beam carrying a slidableweight. The weight is moved until rotation of the specimen under the tool causes the tool to cut as wide a groove as possible in the coating without exposing the metal. .Hardness is calculated as 00 times the load, in grams, divided 'by the width of the groove, in mils. From a study of the film-forming vehicles of the more satisfactory commercial synthetic enamel coatings, it has been determined that they have usually had, at best, shear-hardness values of about 400 to 500 by the Taber method. Significantly improved coatings should have shear-hardness values of at least 550.

It is among the objects of the invention to provide a composition of matter, for use as a flimforming base in synthetic baking enamels, which passes the flexibility and adherence tests outlined above, and which has a shear-hardness of at least 550, when applied as a coating and baked on metal It is a related object to provide such a composition containing ethyl cellulose as a filmforming constituent and having a high resistance to strong solutions of caustic alkalies.

It has now been found that the foregoing and related objects may be attained through the preparation and use of a particular narrow range of proportions of a particular type of ethyl cellulose, a particular class of ureaor melamine-formaldehyde resins, and a specific plasticizer.

The ethyl cellulose which may be used to form the new compositions is one which is soluble in organic solvents, such as toluene and ethanol, to form clear solutions, and which contains from about 45 to about 46.5 per cent ethoxyl groups. A more highly etherified product cannot be used in the present invention, as the resulting compositions are too soft. The appropriate ethyl cellulose is a commercially available product now known as medium ethoxy" ethyl cellulose. The ethyl cellulose for use in the invention is further characterized by being of the low viscosity type, i. e. one whose 5 per cent solution in a 60:40 volume mixture of toluene ethanol has a viscosity of 7 to 50 centipoises. I

The resin constituent of the new composition is a urea-formaldehyde or a melamine-formaldehyde or a ternary ureamelamine-formaldehyde resin of the type which is soluble in organic solvents such as a mixture of toluene and ethanol. This definition serves to distinguish clearly between the resins which are operative to give the desired properties in the new composition and those which are not. If a melamineor ureaformaldehyde resin does not dissolve in tolueneethanol mixtures, it is not intended for use in the new compositions. Numerous resins are commercially available which may be used, and which conform to the above definition, and, while a few of them are named below purely by way of illustration, the manufacturers of urea and melamine resins can at all times supply others which are soluble in toluene-ethanol and which will be equivalent, for present purposes, to the ones here named. Examples of typical operative resins, together with the names of the manufacturers of these and other resins of the class defined, are: "Beetle 227-8, "Beetle" 230-8 and "Melmac 245-8, American Cyanamid and Chemical Corporation; Beckamine" P-l38, Reichhold Chemicals, Inc.; "Uformite MU-56, "Uformite F-226, and Uformite" F-200, The Resinous Products and Chemical Company.

The Beetle resins are condensation products of urea-formaldehyde with butanol and other a1- cohols, with and without alkyd modifiers. They are available chiefly as solutions in combinations of xylene and butanol. The Melmac resins are condensation products of melamine-formaldehyde with butanol and other alcohols, with and without alkyd modifiers. They are sold as solutions in mixtures of xylene and butanol and other special solvents. Beckamine P-138 is a ureaformaldehyde resin supplied at about 58 to 60 per cent concentration in solution in xylenebutanol and having a tolerance for 200 per cent dilution with mineral spirits. The Uformite" resins F200 and F226 are urea-formaldehyde condensates made by chemical combination with an alcohol which, in the case of F226, is capryl alcohol. Uformite" MU56 is a melamine-formaldehyde-butanol resin supplied in the urea-modified form. The Uformite resins are completely miscible with ethanol, butanol, ethyl acetate, butyl acetate, and methyl ethyl ketone.

The third constituent of the new composition is 4,4'-isopropylidene bis-(1-phenoxy-2-propa- 1101). This material is prepared by reacting 4,4- isopropylidene-bis-phenol with propylene chlorohydrin, and may contain some of the isomeric product 4,4'-isopropylidene bis-(2-phenoxy-1- propanol) and it is possible that both permutations ofthe propanol grouping may occur, one at each end of the same molecule. The very viscous liquid product has a boiling range of 240 to 243 C. at 3 millimeters of mercury, absolute pressure.

Only a few compositions of the ternary mixtu e about 150 C. and for about 60 minutes.

of the identified constituents will form liquid coating compositions having the desired combination of properties. These compositions deflne a small area which is shaded and designated as area A on the ternary composition chart constituting the single figure of the accompanying drawing. In the said figure, area A has the approximate shape of a last-quarter-moon, with the tips located at points representing compositions of about 1'7 per cent ethyl cellulose, 30 per cent resin, balance third constituent, and 57 per cent ethyl cellulose, 9 per cent resin, balance thirdconstituent, respectively. The center of the arc connecting these tips is a point representing a composition of about 37 per cent ethyl cellulose, 13 per cent resin, per cent third constituent. Similarly, the center of the concave chord between the tips of area A is a point representing compositions of about 37 per cent ethyl cellulose, 17 per cent resin and 46 per cent third constituent. For simplicity, the term "third constituent" has been used above to denote 4,4'-isopropylidene-bis-(l-phenoxy-Z-propanol), and the term "resin" has been used to designate the particular class of resins defined above. The restricted range of area A is emphasized by the fact that compositions represented by points numbered 1 to 19, inclusive, on the accompanying drawing, all failed in one or more particulars to pass the tests for flexibility, adherence and hardness of baked coatings on metal, while all compositions represented by points within area A met the requirements of these tests.

In preparing the new coating compositions, the three identified constituents are combined in a proportion represented by a point within area A, and are dissolved in a mutual solvent for the three constituents. The resulting lacquer may be employed to deposit clear coatings, or it may be modified by inclusion of pigments to provide the opacity usually desired in synthetic enamel type of coatings. After being applied as a coating on metal, the volatile solvent constituents are evaporated and the coating is baked, preferably at larly, but with less consistently satisfactory results, baking schedules of about 175 C. for 30-45 minutes or about 110 C. for 90-120 minutes may be used.

The ratio of solvent to solute in the new coating compositions is selected so as to provide the desired viscosity for the intended method of coating, and the required solids for the desired coating thickness, as is well known in the art.

The following table defines various compositions falling within the scope of the present invention which meet the above-stated requirements. For comparison and contrast numerous other compositions of the same constituents are listed which fail in one or more respects to pass the tests. All of the compositions were made up to a 20 to 30 per cent content of the three named constituents in a solvent consisting of 80 parts by volume of toluene and 20 parts of ethanol, and were sprayed on clean metal surfaces to deposit coatings from 2 to 3 mils thick (after drying). The coated panels were air dried for 30 minutes, baked at 150 C. for minutes and were then aged at F. and 50 per cent relative humidity for about 18 hours before testing. The ethyl cellulose used in the comparative tests here reported had a viscosity (5 per cent in 60:40 toluene-ethanol) of 10 centipoises and an ethoxyl content of 45.5 per cent. The resin used was a melamine-urea-formaldehyde resin, soluble in Simitoluene alcohol mixtures, manufactured by The Resinous Products and Chemical Company, and sold under the trade-mark designation Uformite MU-56.

6 limited compatibility in the ternary mixture. Similarly, compositions in which the bis-phenol ether of dipropylene glycol, 4,4'-isopropylidenebis-(1-phenoxy-2-(l propoxy) propanol) was M'figoproy one Point Number Ethyl Ceuu' Resin b -(2-phen- Shear-Hardiose PM, Adhesion Flexibility m,

panel) 40 60 [00 7M 508 3 5 t2 0 75 25 5 35 60 495 30 60 480 10 50 40 500 30 55 508 15 34 51 471 15 40 45 360 15 45 40 '542 20 60 423 20 55 565 20 80 50 635 20 4o. 40 High 25 20 55 .532 15 55 554 30 17.5 v 52.5 556 30 20 50 677 10 55 500 35 15 50 M3 35 17 48 653 35 19 46 717 20 40 688 40 40 I) High 42 13.5 44.4 657 45 10 45 608 45 14 41 697 10 40 652 10 35 800 In addition to the properties reported in the table, it was found that all of the metal test panels coated with compositions represented by points falling within area A on the drawing and baked as above described, were highly resistant to attack by strong caustic alkali solutions. No efiect on the coating was noted when 50 per cent aqueous caustic soda solution was allowed to stand thereon at 70 F. for over a week.v

Pigments which may be used in the present composition include all of the usual opacifying agents for synthetic baking enamels, including but not limited to titanium dioxide, zinc, barium and other metallic oxides. and the like.

Comparable tests, made with compositions containing medium ethoxy ethyl cellulose having ethoxy contents variously in the range from 45 to 46.5 per cent, the plasticizer here concemed, and various urea-formaldehyde, melamine-formaldehyde and urea-melamine-i'ormaldehyde resins which are soluble in an 80:20 mixture of toluene and ethanol, have all given comparable results to those reported in the foregoing table. The range of compositions exhibiting the desired combination of adhesion, flexibility and shear-hardness was in each case substantially identical with the range constituting area A in the drawing. When the compositions contained pigments in the usual opacifying concentrations, the baked coatings were often somewhat harder than corresponding unplgmented coatings, but the adhesion and flexibility ratings remained substantially unchanged. When, however, attempts were made to substitute other glycol ethersoi bis-phenols for the 4,4'-isopropylidene bis (1 phenoxy il-propanol) in otherwise comparable compositions, the results were generally unsatisfactory. Thus. compositions of the present type could not be prepared satisfactorily from 4,4'-isopropylidene-bis-phenary-ethanol because this material has very employed, gave inconsistent results. Others of the series of bis-phenol ethers of glycols were incompatible in compositions of the present type. The consistently useful compositions are the ones previousiy described herein.

We claim:

1. A liquid coating composition whereof the essential fllm-formingconstituents are (1) an ethyl cellulose which is soluble in organic solvents, has an ethoxyl content from about 45 to about 46.5, and whose 5 per cent solution in'a mixture or parts by volume of toluene and 40 parts of ethanol has a viscosity in the range from about 7 to about 50 centipoises; (2) a resin selected from the class consisting of those ureaformaldehyde, melamine-formaldehyde and ternary urea-melamine-formaldehyde resins which are soluble in, an 80:20 mixture. by volume, of v toluene and ethanol; and (3) 4,4'-isopropylidene- REFERENCES CITED The following references are of record in the his of this patent:

UNITED STATES PATENTS Number Name Date 2,243,185 Bacon et al. May 27. 1941 3,831,385 Coleman et al. Oct. 5, 1941 

